X-bar, R and S control charts
Monitors the mean (X-bar) and spread (range R or standard deviation S) of a process measured in subgroups, with control limits at ±3 sigma and the SPSS control rules (points beyond 3 sigma, runs of 8, trends of 6, 2 of 3 beyond 2 sigma, etc.).
Menu: Graphs → More charts → Control chart: X-bar, R, S…
When should I use it?
- Checking whether a production or service process is stable over time.
What are the assumptions?
- Subgroups of 2 or more measurements taken in time order; roughly normal measurements.
How do I run it in StatLab?
- Choose the measurement and the subgroup variable.
- Choose R (small subgroups) or S (larger subgroups).
- Keep the control rules switched on.
How do I read the output?
| Output | Meaning |
|---|---|
| Center line | Grand mean of all measurements. |
| UCL / LCL | Center ± 3 sigma / √n (vary with subgroup size). |
| Red squares | Points violating a rule; see the Rule Violations table. |
Worked example
On the practice survey dataset (File → Sample dataset), this command — which the dialog's Paste button also produces — runs the procedure:
PROC vizXbar /y=score /x=group /chart=r /sigmas=3 /rules=YES.
Output tables: Control Limits · Rule Violations for X-bar · Rule Violations for Range.
Computed by StatLab's validated procedure code when this page was built. The example only illustrates the steps; choose variables that fit your own research question.
What are common mistakes?
- Confusing control limits with specification (tolerance) limits.
- Recalculating limits after every out-of-control point.